Inorganic compounds

ABSTRACT

A niobium suboxide powder comprising niobium suboxide particles having a bulk nitrogen content of between 500 to 20,000 ppm. The nitrogen is distributed in the bulk of the powder particles. The nitrogen at least partly is present in the form of at least one of Nb 2 N crystals or niobium oxynitride crystals.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of application Ser. No. 11/916,125,filed on Feb. 18, 2008, now U.S. Pat. No. 8,187,567 which is a U.S.National Phase application under 35 U.S.C. §371 of InternationalApplication No. PCT/EP2006/005184, filed on May 31, 2006 and whichclaims benefit to United Kingdom Patent Application No. GB 0511321.2,filed on Jun. 3, 2005 and to United Kingdom Patent Application No.0602330.3, filed on Feb. 6, 2006. The International Application waspublished in English on Dec. 7, 2006 as WO 2006/128687 A2 under PCTArticle 21(2).

FIELD

The present invention concerns a niobium suboxide powder which is usefulfor the manufacture of solid electrolyte capacitors, particularly anitrogen containing niobium suboxide powder.

BACKGROUND

Solid electrolyte capacitors useful in mobile communication devicesgenerally comprise an electrically conductive carrier of high specificsurface, covered by a non-conductive niobium or tantalum pentoxide layertaking advantage from the high stability and high dielectric constant ofthe valve metal oxide, wherein the isolating pentoxide layer can begenerated by electrolytic oxidation at very constant thickness. Thevalve metal or conductive lower oxides (suboxides, NbO_(x)) of the valvemetals are used as the carrier material. The carrier, which forms one ofthe electrodes (anode) of the capacitor generally has a highly poroussponge-like structure which is generated by sintering of very fineprimary structures or sponge-like secondary structures. The surface ofthe conductive carrier structure is electrolytically oxidized(“forming”), whereby the thickness of the isolating pentoxide layer isdetermined by the maximum voltage of the electrolytic oxidation(“forming voltage”). The counter electrode is generated by soaking ofthe sponge-like surface-oxidized structure with manganese nitrate, whichis thermally transformed into manganese dioxide, or, by soaking of aliquid precursor of a polymer electrolyte (e.g. PEDT, polypyrole) andpolymerisation thereof. Electrical terminals are a tantalum or niobiumwire sintered with the sponge-like structure at the anode side and themetallic housing of the capacitor, which is isolated against the wire atthe cathode side.

The capacitance C of the capacitor is calculated according to theformulaC=(F·∈)/(d·V _(F)),wherein F is the active surface of the capacitor, E is the dielectricconstant of the pentoxide layer, d is the thickness of the isolatingpentoxide layer per Volt forming voltage, and V_(F) is the formingvoltage. The ratio ∈/d is nearly equal for tantalum pentoxide andniobium pentoxide (1.64 resp. 1.69), although ∈ (27.6 resp. 41) and d(16.6 resp. 25 A/V) differ appreciably. Accordingly, capacitors on basisof both the pentoxides having the same geometrical structure have thesame capacitance. Specific capacitances per weight differ due to thedifferent densities of Nb, NbO_(x) and Ta respectively. Carrier (anode)structures of Nb or NbO_(x), accordingly, do have the advantage ofsaving weight, when used in mobile phones, where reduction of weight isone of the objects. Regarding costs, NbO_(x) is more feasible than Nb,providing part of the volume of the anode structure from oxygen.

An important quality criterion is life time of the capacitor, whichdepends from the voltage of operation thereof and decreases withincreasing voltage. For opening up a wider range of applications, itwould be desirable to increase the lifetime, particularly in the uppervoltage of operation level.

Furthermore it would be desirable to allow for an increase of thetemperature of operation. Presently, the temperature of operation ofcapacitors based on NbO is limited to about 125° C. A higher allowabletemperature of operation would open up the use of capacitors on basis ofNbO in the automotive industry.

Furthermore, with reference to safety aspects, it would be desirable toincrease the breakdown voltage, and to slow down the burning rate, andto reduce the generation of heat during burning after ignition, of thepowders, the sintered anode structures and of the capacitors.

SUMMARY

One object of the invention is to provide a niobium suboxide powder ofimproved properties from which capacitors of increased service life timemay be produced.

Another object of the invention is to provide a niobium suboxide powderof improved properties allowing for higher temperature of operation ofcapacitors made there from.

Another object of the invention is to provide a niobium suboxide powderof improved properties allowing for the production of capacitors ofincreased breakdown voltage.

Another object of the invention to provide a niobium suboxide powder,and an anode structure made there from, with reduced burning rate andreduced generation of heat, when ignited.

These and other objects are achieved with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in greater detail below on the basisof embodiments and of the drawings in which:

FIG. 1 shows the x-ray diffraction pattern of the powder of the presentinvention produced in example 2;

FIG. 2 shows the x-ray diffraction pattern of the powder of the presentinvention produced in example 4;

FIG. 3 shows the x-ray diffraction pattern of the powder of the presentinvention produced in example 5;

FIG. 4 shows respective curves for the powder produced in comparativeexample 1;

FIG. 5 shows the respective curves for the powder of the presentinvention produced in example 2;

FIGS. 6 a and 6 b show the leakage current respectively the capacitanceof a capacitor made from the powder of comparative example 1 at atemperature of 125° C. and a working voltage of 4 V during 5,000 hoursof operation;

FIGS. 7 a and 7 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 2 (N-doped) at atemperature of 125° C. and a working voltage of 4 V during 9,000 hoursof operation;

FIGS. 8 a and 8 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 1 (comparison) at atemperature of 140° C. and a working voltage of 2 V during 5,000 hoursof operation; and

FIGS. 9 a and 9 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 2 (N-doped) at atemperature of 140° C. and a working voltage of 2 V during 5,000 hoursof operation.

DETAILED DESCRIPTION

Subject of the present invention is a niobium suboxide powder comprisingniobium suboxide particles having a bulk nitrogen content of between 500to 20,000 ppm, preferably 1,000 to 10,000 ppm. More preferred is anitrogen content between 2,000 and 8,000 ppm, particularly preferred3,000 to 5,000 ppm.

Preferably the nitrogen is present in the niobium suboxide powderaccording to the invention at least partly in the form of Nb₂N crystalsor niobium oxynitride. NbO_(x)N_(v) crystals.

It is well known in the technology of tantalum capacitors that surfacenitrogen has a positive effect on sintering of tantalum powder, alsoimproving leakage current of tantalum capacitors. Contrary to this, animportant aspect of the present invention is that the nitrogen is quasihomogeneously distributed in the bulk of the powder particles preferablyat least partly in the form of very small Nb₂N crystal domains, in anamount and size sufficiently large that a peak at a 2Θ-angle of about38.5° (101-reflex of Nb₂N) can be detected when investigated by x-raydiffraction method using Cu_(Kα)—radiation.

Preferably, the height of the Nb₂N peak at about 2Θ=38.5° is less than25% of the height of the NbO peak at 2Θ=30° (110-reflex of NbO),particularly less than 15% of the height of the NbO peak at 2Θ=30°.

Furthermore preferred powders show an Cu_(Kα)-x-ray peak at 2Θ=38.5°,the height of which is at least 2%, preferably at least 5%, of theheight of the NbO-peak at 2Θ=30°.

In the higher range of nitrogen content additional crystalline nitridephases such as niobium nitride or niobium oxynitride may be detectable.More specifically, Nb₄N₃, NbN_(0.77), Nb_(0.77)N_(0.091), NbN_(0.64),NbN_(0.9), NbN_(0.95), Nb_(4.62)N_(2.14), Nb₄N_(3.92), Nb₄N₅, Nb₅N₆,NbN_(0.801), NbN etc or mixtures thereof, or niobium oxynitrides, likeNbN_(0.6)O_(0.3), NbN_(0.6)O_(0.2), NbN_(0.9)O_(0.1), Nb(N.O) etc, ormixtures thereof with each other or niobium nitrides, may be detectable.In particular, NbN_(0.77), NbN_(0.95), NbN etc, or niobium oxynitride,may be detectable.

The half-value width of the Cu_(Kα1)-peak at about 2Θ=38.5° ((101)-peakof Nb₂N) preferably is between 0.05° and 0.2°, preferably 0.07 and0.15°, as determined with an goniometer type Panalytical X Pert MPD PW3050, anode Cu at 50 kV and 40 mA, having a divergence slit and antiscatter slit of ½°2Θ each, a receiving slit of 0.2 mm, soller slits of0.04 rad, a beam mask of 20 mm, the detector being proportional Xefilled. The scanning program is step size 0.01°2Θ with scan speed of0.001°2Θ/sec between 37.7 and 39.5°2Θ. The Cu_(Kα2) reflex is striped.

Preferably the powder according to the present invention has a grainsize distribution characterized by a D10-value of between 50 and 90 μm,a D50-value of between 150 and 210 μm, and a D90-value of between 250 to350 μm, as determined according to ASTM B 822 (“Mastersizer”, wettingagent Daxad 11). Particularly preferred are powders having spherical orelliptical grains providing for good flowability of less than 80 sec/25g, preferably 60 sec/25 g, particularly preferred 40 sec/25 g, asdetermined according to ASTM B 213 (“Hall flow”). The bulk density ofthe powders according to the invention preferably is between 0.5 and 2g/cm³, preferably 0.9 and 1.2 g/cm³ (14.8 to 19.7 g/inch³), asdetermined according to ASTM B 329 (“Scott density”).

The individual grains or particles of the niobium suboxide powderpreferably are highly porous agglomerates of dense primary particles ofmean size having a smallest cross sectional diameter of 0.1 to 1.5 μm,preferably 0.3 to 1.0 μm. The primary particles may have spherical,chip-like or fibrous structure. Preferably the smallest cross sectionaldiameter of the primary particles is between 0.4 and 1 μm.

The porosity of anodes sintered from the powder according to theinvention, as determined by mercury intrusion, preferably is between 50and 70% by volume, particularly preferred between 53 and 65% by volume.More than 90% of the pore volume consists of pores having a diameter of0.2 to 2 μm. The broad pore distribution curve has steep flanks at bothsides with a minimum in the range of twice the primary particlediameter.

The specific surface area of the powders according to the inventionpreferably is between 0.5 and 12.0 m²/g, preferably 0.6 to 6 m²/g, morepreferably 0.7 to 2.5 m²/g, as determined according to ASTM D 3663(“BET-surface”), particularly preferred is a specific surface of between0.8 and 1.2 m²/g or of between 0.8 and 1.3 m²/g.

Capacitors made from the powder according to the invention may have aspecific capacity of between 40,000 to 300,000 μFV/g, commonly between60,000 and 200,000 μFV/g.

Preferred niobium oxide powders according to the invention have acomposition according to the formula NbO_(x) with 0.7<x<1.3,corresponding to an oxygen content of between 10.8 and 18.3% by weight,particularly preferred is 1.0<x<1.033, or powders having an oxygencontent of between 14.5 to 15.1% by weight.

Generally, impurities in the niobium suboxide powders according to theinvention should be as low as possible, particularly harmful impuritiesin capacitor application such as Fe, Cr, Ni, Cu, Na, K, and Cl, are lessthan 15 ppm each. Preferably the sum of those harmful impurities is lessthan 35 ppm. The carbon content preferably is less than 40 ppm. Otherless harmful impurities such as Al, B, Ca, Mn, and Ti are preferablypresent in an amount of less than 10 ppm, Si less than 20 ppm. Mg maypresent in an amount of up to 500 ppm.

Phosphorous generally is not harmful. In niobium metal and tantalummetal powders for capacitors, phosphorous doping is used for reducingthe sintering activity of the powders. A reduction of sintering activityof the niobium suboxide powders according to the invention is normallynot desirable. Preferably the phosphorous content accordingly is below10 ppm. If necessary the substantially phosphorous free powders maytreated with phosphorous acid, ammonium hydrogen phosphate or ammoniumphosphate solution prior to sintering.

Tantalum may be present as an alloying component substituting niobiumaccording to formula (Nb,Ta)O_(x).

Subject of the invention also is a process for the manufacture ofnitrogen containing niobium suboxide powder, which process starts from aniobium metal powder precursor and is characterized in that the niobiummetal precursor is nitrided before transformation to niobium oxide.

Various methods are known for the transformation of niobium metal powderinto NbO. The preferred method according to the invention is the solidstate disproportionation method: The niobium metal powder is mixed witha stoichiometric amount of niobium oxide, which is oxidized higher thanthe desired product, preferably Nb₂O₅ or NbO₂, and thereafter themixture is heated to a temperature sufficient to initiatedisproportionation, generally to a temperature between 800 and 1600° C.in a non-oxidizing atmosphere, preferably a reducing inert gasatmosphere such as hydrogen or argon/hydrogen mixtures, for a timesufficient to give a homogeneous oxygen distribution, e.g. for severalhours. Preferably, the metal precursor as well as the oxide precursorconsists of primary particles of about 1 μm diameter or less (smallestcross section, if non-spherical).

For the nitridation of the niobium metal precursor powder (doping of themetal with nitrogen) the metal powder is mixed with a solid nitrogencontaining compound, such as Mg(N₃)₂ or NH₄Cl, or treated with anaqueous solution thereof, and heated to a temperature of 400 to 750° C.in an inert atmosphere, or reacted with a gaseous nitrogen containingreactant, such as N₂ or NH₃ at a temperature of 400 to 750° C.Preferably the gaseous reactant is supplied in an inert gas atmosphere,such as argon, at a ratio of 15 to 30%. The amount of nitrogen doping iscontrolled by properly selecting time and temperature of the heattreatment.

According to another method, nanocrystalline niobium nitride may bemixed at the required ratio with niobium metal powder and heat treatedat between 400 and 900° C. in an inert gas atmosphere for nitridation ofthe metal powder.

The niobium metal powder precursor and the higher oxidized oxideprecursor may be mixed prior to the nitridation, which allows forreduction of handling. In this case, after completion of thenitridation, the atmosphere is exchanged and the mixture is furtherheated to the temperature where the solid state disproportionationoccurs.

Extremely pure Nb₂O₅, which may be used as the oxide precursor of theinvention, is available from precipitation of niobium hydroxide from anaqueous H₂NbF₇ solution by addition of an aqueous NH₄OH solution andcalcinations of the niobium hydroxide separated from the solution.

The niobium metal precursor preferably is obtained from extremely pureNb₂O₅ by reduction. This may occurs by aluminothermic reduction, i.e.igniting a Nb₂O₅/Al mixture, washing out the aluminium oxide there fromand purification of the niobium metal by electron beam heating. Theniobium metal ingot obtained thereby may be made brittle by diffusion ofhydrogen in a known manner and milled to give a powder having chip likeparticle shape.

The preferred process to reduce the pentoxide to metal is the two-stageprocess disclosed in WO 00/67936. According to this process thepentoxide is first reduced to approximately niobium dioxide in hydrogenatmosphere at about 1,000 to 1,600° C. and in the second stage toniobium metal with magnesium vapour at about 900 to 1,100° C. Magnesiumoxide, which is formed during reduction, may be removed by washing withacid. However it is not necessary to remove the magnesium oxide prior tonitridation and transformation of the niobium metal to NbO_(x). In thecontrary, the presence of magnesium oxide during the transformation toNbO_(x) has a positive influence on the porosity of the NbO_(x)-powder.

The grain size (secondary particle size) of the powder particles may beadjusted by properly selecting the temperature at which the solid statedisproportionation is carried out or later by a sintering heat treatmentof the product in an argon atmosphere preferably containing up to 10% ofhydrogen, and screening.

The invention is now explained in more detail by way of the followingexamples:

A Precursors: The following precursors were used:

Al: High purity Nb₂O₅ obtained by precipitation from an aqueous H₂NbO₇solution by addition of an aqueous NH₄OH solution, separation of theprecipitate, drying and calcination in air at 1,100° C., with thefollowing analytical data:

Al: 1 ppm

Cr: <0.3 ppm

C: <10 pp

Fe: <0.5 ppm

K: 0.6 ppm

Mg: <1 ppm

Mn: <0.1 ppm

Mo: <0.3 ppm

Na: 3 ppm

Ni: <0.2 ppm

Si: 14 ppm

Scott density: 12.2 g/inch³.

A2: NbO₂ obtained from reduction of precursor Al (Nb₂O₅) in a molybdenumcrucible in hydrogen at 1450° C. with the following analytical data:

Al: 2 ppm

Cr: <2 ppm

C 12 ppm

Fe: <2 ppm

K: 1 ppm

Mo: 54 ppm

Na: 4 ppm

Ni: <2 ppm

N: <300 ppm

O: 26.79%

Si: 14 ppm

BET: 0.17 m²/g

Scott density: 23.6 g/inch³

A3: Niobium metal: The precursor A2 (NbO₂) is placed within a reactor ona sieve made from niobium wire. Below the sieve is a crucible containing1.05 times the stoichiometric amount magnesium with reference to theoxygen content of the NbO₂. Argon is continuously introduced at thebottom of the reactor and removed from the reactor on top. Then thereactor is heated to about 950° C. After consumption of the magnesiumthe reactor is cooled down to about 150° C. and air is slowly introducedinto the reactor to passivate the niobium metal surface with thefollowing analytical data:

Al: 2 ppm

Cr: <2 ppm

C <10 ppm

Fe: <2 ppm

K: 1 ppm

Mg: 28.14%

Mo: 41 ppm

Na: 2 ppm

Ni: <2 ppm

N: <300 ppm

O: 18.74%

Si: 7 ppm

A4: Niobium metal obtained by washing precursor A3 (magnesium oxidecontaining niobium metal) with sulphuric acid and rinsed with wateruntil neutral. The analytical data are as follows:

Al: 3 ppm

Cr: <2 ppm

C <10 ppm

Fe: <2 ppm

K: 1 ppm

H: 344 ppm

Mg: 750 ppm

Mo: 75 ppm

Na: 3 ppm

Ni: <2 ppm

N: <300 ppm

O: 1.65%

Si: 8 ppm

BET: 4.52 m²/g

If “<” is presented in the analytical data, the respective content isbelow the analytical limit and the figure behind represents theanalytical limit.

POWDER PRODUCTION EXAMPLES Example 1 Comparison

53.98 weight-% of precursor A4 (Nb) and 46.02 weight-% of precursor Al(Nb₂O₅) are homogeneously mixed and heated in a hydrogen atmosphere to1,400° C. The product properties are shown in table 1.

Example 2

Precursor A4 (Nb) is homogeneously mixed with 1.5 times thestoichiometric amount of magnesium (with reference to the oxygencontent) and 5.4 parts by weight of NH₄Cl (per 100 parts Nb) and placedin reactor. The reactor is then rinsed with argon and heated to 700° C.for 90 minutes. After cooling down the reactor is slowly filled with airfor passivation. After washing with sulphuric acid and rinsing anitrogen doped niobium metal has been obtained, containing between 9,600and 10,500 ppm nitrogen (average 9871 ppm). The oxygen content is 6724ppm.

The nitrogen doped niobium is transformed to NbO in the same manner asin example 1. The product properties are shown in table 1. The x-raydiffraction pattern of the powder is shown in FIG. 1. Clearly, the Nb₂N(101)-peak at 2Θ=38.5° indicated by the arrow can be recognized.Accordingly, at least part of the N-doping is present in the form of acrystalline Nb₂N phase.

Example 3

Example 2 was repeated with the deviation that the addition of NH₄Cl wasincreased to 8.2 parts by weight. The niobium powder has an averagenitrogen content of 14730 ppm. The oxygen content is 6538 ppm. Thesuboxide product properties are shown in table 1.

Example 4

53.95 parts by weight of precursor A4 (Nb) and 46.05 parts by weight ofprecursor Al (Nb₂O₅) are mixed homogeneously and placed in a reactor.The reactor was rinsed with argon and heated to 500° C. Thereafter thereactor was three times with an 80% Ar/20% N-mixture for 30 minutes eachtime. Thereafter powder mixture is heated to 1450° C. in hydrogenatmosphere. The product properties are shown in table 1. The x-raydiffraction pattern of the powder is shown in FIG. 2. Clearly, theNb₂N-(101)-peak indicated by the arrow at 2Θ=38.5° can be recognized.

Example 5

Precursor A3 (MgO containing Nb) is nitrided with nitrogen gas at 630°C. and thereafter magnesium oxide and residual magnesium metal removedby washing with 15% sulphuric acid. The oxygen content of the resultingniobium metal is 1.6% b.w.; the nitrogen content is 8515 ppm.

56.03 parts by weight of the N-doped niobium metal and 43.97 parts byweight of precursor Al (Nb₂O₅) are mixed homogeneously and heated to1,100° C. in a hydrogen atmosphere. The product properties are shown intable 1. The x-ray diffraction pattern of the powder is shown in FIG. 3.Clearly, the Nb₂N-(101)-peak at 2Θ=38.5° can be recognized.

TABLE 1 Properties of NbO_(x) powders Analysis Exam- Mastersizer ScottHall O ple BET D10 D50 D90 density flow % N No. m²/g μm μm μm g/inch³sec b.w. ppm 1 1.02 60.52 190.63 295.4 15.8 48 14.97 <300 (Comp) 2 1.0462.06 170.67 290.05 17.2 43 14.9 5848 3 1.03 59.73 185.54 270.76 16.7 5414.93 8115 4 1.0 58.73 191.04 299.93 14.7 45 14.98 6281 5 2.31 52.76150.46 268.37 15.2 61 14.94 5062

Example 6

Precursor A2 (NbO₂) is placed within a reactor on a sieve made fromniobium wire. Below the sieve is a crucible containing 1.05 times thestoichiometric amount magnesium with reference to the oxygen content ofthe NbO₂. Argon is continuously introduced at the bottom of the reactorand removed from the reactor on top. Then the reactor is heated to about950° C. After consumption of the magnesium the reactor is cooled down to575° C. and nitrogen is introduced for 3 hours. After cooling down,passivation and removal of magnesium oxide and residual magnesium metala nitrogen doped niobium metal is obtained, which can be used fortransformation to NbO.

Investigation of the Burning Rate

50 g of each powders of examples 1(comparison), 2 and 3 were arranged ona niobium sheet of 0.1 mm thickness in an array of 150×30 mm. The powderarrays were ignited at one end and the time for complete burning wasmeasured (in air):

powder of example 1 (comparison): burning time 3 min 35 sec,

powder of example 2 burning time 6 min 25 sec,

powder of example 3 burning time 8 min 10 sec.

DSC/TGA Investigation

A sample of example 1 and a sample of example 2 were heated in air from25 to 600° C. and the increase of weight measured by thermo gravimetry(TGA). Simultaneously the heat flow accompanied therewith was measuredby the DSC method. FIG. 4 shows respective curves for the powder ofexample 1 (comparison) and FIG. 5 shows the respective curves for thepowder of example 2. In those Figs. curve A indicates the temperature(left inner scale from 0 to 600° C.), curve B indicates weight-% (leftouter scale from 95 to 125%), and curve C indicates the heat flow withcorrection for weight (right scale from 0 to 120 W/g) over time(horizontal scale from 0 to 50 resp. 60 sec.), each. Both samples show aslight increase in weight above about 200° C. with small heatdevelopment. Until about 450° C. weight increase and exothermic heat isvery similar for both samples. Above about 450° C. the nitrogen freesample suffers a sudden increase in weight and corresponding strongdevelopment of heat (FIG. 5), whereas for the nitrogen containing sampleheat development and weight increase rate remain moderate also above450° C. with no exothermic peak.

Preparation of Anodes

The NbO_(x) powder of example 1 (comparison) and example 2 respectivelyare filled into cylindrical press moulds of 4.1 mm diameter and 4.2 mmlength around an axially arranged tantalum wire. The powder is pressedto green bodies having a density of 2.8 g/cm³. The green bodies wereplaced on a niobium tablet and heated to 1460° C. in a vacuum of 10⁻⁸bars for a holding time of 20 minutes.

Investigation of the Break Down Voltage of Anodes

The anodes are immersed into an aqueous 0.1% phosphoric acid solution(conductivity 8,600 μS/cm) at a temperature of 85° C. and a constantcurrent of 150 mA is applied for forming until voltage suddenly dropsdown (break down voltage). The anodes made from powder of example 1(comparison) gave a sudden voltage drop at 96 V, whereas the anodes madefrom powder of example 2 gave a sudden voltage drop at 104 V.

Investigation of Capacitors

In an industrial production line capacitors were produced from thepowder of example 1 (comparison) as well as from powders of example 2.The powders are pressed in pressing moulds of 4.2 mm diameter and 4.1 mmlength around a centrally arranged tantalum wire at press density of 2.8g/cm³. The green bodies were sintered in a vacuum of 10⁻⁸ bars. Theanode structures are anodised to a forming voltage of 16 V and providedwith a MnO₂-cathode. The anodes are operated at constant temperature andwith an alternating current of the working voltage as presentedhereafter. 50 capacitors were run in parallel in each of the followingtests:

FIGS. 6 a and 6 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 1 (comparison) at atemperature of 125° C. and a working voltage of 4 V during 5,000 hoursof operation.

FIGS. 7 a and 7 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 2 (N-doped) at atemperature of 125° C. and a working voltage of 4 V during 9,000 hoursof operation.

FIGS. 8 a and 8 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 1 (comparison) at atemperature of 140° C. and a working voltage of 2 V during 5,000 hoursof operation.

FIGS. 9 a and 9 b show the leakage current respectively the capacitanceof a capacitor made from the powder of example 2 (N-doped) at atemperature of 140° C. and a working voltage of 2 V during 5,000 hoursof operation.

What is claimed is:
 1. A niobium suboxide powder comprising niobiumsuboxide particles having a bulk nitrogen content of between 500 to20,000 ppm, wherein the nitrogen is distributed in the bulk of thepowder particles, the nitrogen is at least partly present in the form ofNb₂N crystals, the Nb₂N crystals have a size sufficiently to give a peakin Cu_(Kα)x-ray radiation at 2Θ-angle of about 38.5°, and a height ofthe Nb₂N peak at about 2Θ=38.5° is between 2 to 25% of a height of anNbO peak at 2Θ=30°.
 2. The niobium suboxide powder according to claim 1,wherein the nitrogen content is between 1,000 to 8,000 ppm.
 3. Theniobium suboxide powder according to claim 1, wherein the nitrogencontent is between 3,000 to 5,000 ppm.
 4. The niobium suboxide powderaccording to claim 1, wherein the Cu_(Kai)-peak at about 2Θ=38.5° has ahalf-value width of between 0.05 and 0.2°.
 5. The niobium suboxidepowder according to claim 1, wherein the powder has a grain sizedistribution characterized by a D10-value of between 50 and 90 μm, aD50-value of between 150 and 210 μm, and a D90-value of between 250 to350 μm, as determined according to ASTM B
 822. 6. The niobium suboxidepowder according to claim 1, wherein the niobium suboxide powderparticles are agglomerates of primary particles of mean diameter of 0.1to 1.5 μm.
 7. The niobium suboxide powder according to claim 1, whereinthe niobium suboxide powder particles are agglomerates of primaryparticles of mean diameter of 0.3 to 1.0 μm.
 8. The niobium suboxidepowder according to claim 1, wherein the niobium suboxide has thecomposition NbO_(x) with 0.7<x<1.3.
 9. The niobium suboxide powderaccording to claim 8, wherein 1<x<1.033.
 10. The niobium suboxide powderaccording to claim 1, wherein the oxygen content is between 14.5 to 15.1wt-%.
 11. The niobium suboxide powder according to claim 1, having aburning time of more than 5 min, when 50 g thereof are arranged in anarea of 150×30 mm on a niobium sheet of 0.1 mm thickness and ignited atone end.
 12. An electrolyte capacitor which comprises the niobiumsuboxide powder according to claim 1.